1. Field of the Invention
This invention relates generally to improving the electrical performance of active semiconductor devices with defects. More particularly, it relates to improving the electrical performance of lattice-mismatched semiconductor devices used in optical cavities, such as vertical cavity surface emitting lasers and vertical lasing semiconductor optical amplifiers.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of data over a telephone company""s trunk lines, the transmission of images or video over the Internet, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth capacity will only increase.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the SONET protocol have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal. Optical amplifiers, which boost the power of the optical signal propagating through the system, are also a basic building block for fiber communications systems.
Many technologies exist for building the various components within fiber communications systems. For example, optical amplifiers may be created by doping a length of fiber with rare-earth metals, such as erbium, in order to form an active gain medium. The doped fiber is pumped to create a population inversion in the active medium. Then, when an optical signal propagates through the doped fiber, it is amplified due to stimulated emission. Transmitters may be a laser source modulated externally by a lithium niobate modulator, or a directly modulated DFB laser or FP laser. Semiconductor technologies are another alternative for building components. For example, laser sources may be created by using semiconductor fabrication techniques to build an optical cavity and a semiconductor active region within the optical cavity. The active region acts as the gain medium for the cavity. Pumping the active region results in a lasing optical cavity. In a variation of this theme, semiconductor devices may also be used as amplifiers. In one approach, an electrical current pumps the semiconductor active region of the amplifier, resulting in an increased carrier population. The optical signal then experiences gain as it propagates through the active region due to stimulated emission.
These types of semiconductor devices often will function in both the optical and electrical domains. For example, the amplifier described above functions as an optical device since an optical signal injected into the device is amplified as it propagates through the device. However, it also functions as an electrical device since the basic gain mechanism is an electrical effect (e.g., a p-n junction or multiple quantum well) and the device may also be electrically pumped. As a result, finding materials which can meet all of the optical and electrical requirements is difficult and, more often than not, more than one materials system is used to create the required device. For example, it is common to create devices which combine GaAs-based Bragg mirrors with InP-based active regions. However, the use of multiple materials systems creates its own problems, not the least of which is how to effectively combine the different materials systems. For example, GaAs and InP are not lattice matched and devices which combine the two typically will have a lattice mismatched interface somewhere, which can create undesirable effects. In addition, the techniques used to fabricate these devices may also lead to undesirable effects, such as the creation of deep-level defects, particularly if the device requires unusual or non-standard fabrication processes.
Thus, there is a need for semiconductor devices which overcome the undesirable effects generated by combining different material systems and/or by the processes used to fabricate the devices.
In accordance with the present invention, a monolithic semiconductor device extends primarily along a vertical direction and includes the following layers. A first semiconductor layer is doped either n-type or p-type, but preferably is n-type. A second semiconductor layer is doped with the other doping type (i.e., p-type if the first layer is n-type and vice versa). An active region is formed between the first and second semiconductor layers. A third semiconductor layer is doped the same type as the second semiconductor layer. The second and third semiconductor layers are in direct contact with each other but are lattice mismatched. In an alternate embodiment, the interface between the two layers is characterized by trap-like defects rather than being lattice mismatched. An additional doping layer, preferably a delta doping layer, is located in close proximity to the interface between the second and third semiconductor layers. The dopant concentration in the doping layer is heavier than in the bulk of either the second semiconductor layer or the third semiconductor layer. In an alternate embodiment, the second semiconductor layer includes two sublayers with a contaminated interface between them. Thus, one of the sublayers acts as a spacer to separate the contaminated interface from the lattice mismatched (or heavily defective) interface.
In a preferred embodiment, the device further includes a bottom mirror layer and a top mirror layer, which together form a laser cavity which includes the active region. In one embodiment, the laser cavity is designed to output laser light in a vertical direction, for example as a vertical cavity surface emitting laser (VCSEL). In another embodiment, the device includes a pump input to the active region. The active region also includes an amplifying path which is not colinear with the optical path of the laser cavity. When the active region is pumped above a lasing threshold for the laser cavity, the gain of the active region is clamped to a gain value which essentially is constant. Thus, an optical signal propagating along the amplifying path is amplified by an essentially constant gain.
The present invention is particularly advantageous for a number of reasons. The doping and spacer layers both lead to improved electrical performance. For example, in the context of the VCSELs and VLSOAs, they result in lower turn-on voltages for the laser cavities, thus resulting in lower power consumption and lower heat generation. They also mitigate nonlinear thyristor-type effects caused by the lattice-mismatched interface and/or deep-level defects.